1. Field of the Invention
The present invention relates to providing light for non-self illuminating displays, and more particularly, to a backlight unit.
2. Discussion of the Related Art
In the past, Cathode Ray Tubes (CRTs) were generally used as display devices. For example, CRTs were used in TVs and as monitors in measuring instruments and information terminals. However, a CRT is heavy and has a large size due to the glass tube for generating an image. Consequently, the CRT is not meeting the size and weight requirements in the trend of making electronic products smaller and lighter. The CRT is now being replaced by flat panel devices, such as the Liquid Crystal Display Device (LCD), Plasma Display Panels (PDP), Electro-luminescence Display Device (ELD).
Liquid crystal display devices have been replacing CRTs because they have the advantages of small size, light weight, and low power consumption. Liquid crystal display devices have not only been used as monitors for laptop computers, but also as monitors for desktop computers, and in large screen information display devices, and large screen TVs. Accordingly, demands for the liquid crystal display devices are increasing.
Because liquid crystal display devices are not self-luminous, an external light source is required to display a picture. In general, a backlight unit is used as a light source for the liquid crystal display device. The backlight unit has cylindrical lamps arranged at an edge of the liquid crystal display device or directly under the liquid crystal display device.
The edge-type backlight has a lamp unit mounted at an edge of a light plate that guides the light. The lamp unit of an edge-type backlight is provided with a lamp for emitting the light, a lamp holder at opposite ends of the lamp for holding the lamp, and a reflective plate around an outside circumferential surface of the lamp at a side surface of the light plate for reflecting the light from the lamp toward the light plate. The edge-type backlight is mostly used in a relatively small size liquid crystal display devices, such as a monitor for a laptop computer, or a desktop computer. The edge-type backlight has good light uniformity, a long lifetime, and a thin profile.
The direct-type backlight was developed when the size of the liquid crystal display devices started having display screens larger than 20″. The direct-type backlight has a plurality of lamps arranged at regular intervals under a diffusion plate for directing the light toward a front surface of the LCD panel. Since the direct-type backlight has better light efficiency than the edge-type, the direct-type is typically used in large sized liquid crystal display devices.
The direct-type backlight is typically used in liquid crystal display devices, such as a large sized monitor or a TV set, that are used for long periods of time. The direct-type backlight also uses more lamps than an edge-type backlight unit. Thus, the possibility for a lamp failure in the direct-type backlight is greater than for an edge-type backlight. Moreover, in the case of edge-type backlight having the lamp units mounted on opposite edges of the light plate, even if one lamp fails to turn-on, the luminance of the screen drops. However, if one lamp fails in the direct-type, as there are a plurality of lamps mounted on an under of the screen, a portion of the screen where a lamp failed becomes distinctively darker than other portions of the screen. Consequently, replacement of the lamps is desirable in the direct-type backlight and a structure for mounting/dismounting the lamps is also desirable.
A related art backlight unit will be described with reference to the attached drawings. FIG. 1 illustrates a perspective view of a related art direct-type backlight unit, and FIG. 2 illustrates a perspective view showing an inside of an upper member shown in FIG. 1.
Referring to FIGS. 1 and 2, the related art direct below type backlight unit is provided with a plurality of lamps 1 arranged at regular intervals each with external electrodes at opposite ends. A lower member 3 having a plurality of slots at opposite edges receives and supports opposite ends of the lamps 1. A lower reflective plate 4 on the lower member 3 for reflecting light from the lamps 1. Upper members 6 at the opposite to the lower member 3 for holding, and supporting the lamps 1, together with the lower member 3.
The lamps 1 are external electrode florescent lamp (EEFL), and though not shown, the external electrodes at opposite ends of the lamps 1 are connected in common to a common electrode. The upper member 6 for holding the lamps 1 together with the lower member 3 has slots at portions where the lamps 1 are to be placed. There is a rib 8 within the upper member 6. The rib 8 may be provided between each of the lamps 1, or every second, or more than every second lamp. That is, the upper member 6 has an empty space, except for the ribs 8. Further the upper member 6 has air inlet/outlets at opposite ends.
In general, air has a very low thermal conductivity, as low as approximately 0.02 W/mK. The ribs 8 in the upper member 6 disrupt air flow such that the transmission of heat from the electrodes and the common electrode of the lamps 1 to the air. This lack of heat transmission causes hot spots near the electrode portions of the lamps 1. The hot spots cause a drop in optical efficiency in the edge portions of the lamps and thus impairing luminance uniformity of the entire backlight unit. Moreover, despite the air inlet/outlets in the upper member 6, the air has to pass over multiple ends of lamps 1 and go around the ribs before cooling the lamps 1 in the middle of a direct-type backlight. Thus, some lamps may be cooled better than other lamps, which also impairs luminance uniformity of the entire backlight unit. In the related art, cold cathode fluorescent lamps (CCFL) are supported with a lamp holder or the upper member is filled with a solid heat conductive material to improve heat dissipation performance at the electrode portion of the lamps. Such designs are not practicable because the methods cause additional problems, such as increased cost and increased weight.